1. Field of the Invention
This invention relates to an illumination optical system which is capable of efficiently changing the distribution of illumination light having directivity, for example, from a light guide or surface illuminant.
2. Description of Related Art
Light guides have been widely used in illumination optical systems for endoscopes. Although the endoscopes have various field angles in accordance with their specifications, they are generally wide-angle and those having field angles of about 100 to 140.degree. are chiefly used. On the other hand, in the illumination optical systems for endoscopes, light guides with a numerical aperture of approximately 0.6 are often used. Since this is on the order of 70.degree. in terms of an angle of view, it follows that the radiation angle of illumination light is smaller than the field angle of the endoscope. Thus, in order to increase the radiation angle of illumination light from a light guide, many endoscopes are provided with a variety of optical members at the exit end faces of light guides.
For example, in each of Japanese Patent Preliminary Publication Nos. Sho 63-239415 and Hei 8-122661, a negative lens is placed, as an optical member for increasing the radiation angle of illumination light, at the exit end of the light guide. In Japanese Patent Preliminary Publication No. Hei 1-198717, frosted glass is used as another optical member. Further, Japanese Patent Preliminary Publication No. Sho 55-136031 discloses an optical system in which cylindrical or ball lenses are arranged in front of the exit end face of the light guide.
However, illumination lenses disclosed in Sho 63-239415 or Hei 8-122661 are negative lenses, and thus must have considerable thicknesses in order to completely pass a light beam through their periphery as well. Consequently, the outside diameter of the lens will be increased and an intention of reducing the diameter of the endoscope will be defeated. Furthermore, since the shape of the lens is limited to the form of, or close to, a circle, the exit end face of the light guide must also be shaped into a circular form to suit the shape of the lens. In this way, for a light guide having an annular or crescentric shape, it is difficult to place such an illumination lens.
An approach set forth in Hei 1-198717 brings about phenomena that the amount of light for illuminating an area outside the visual field, because of its diffusion, is increased, while the amount of light for illuminating an area inside the visual field is decreased. Hence, there is a problem in illumination efficiency.
The optical system disclosed in Sho 55-136031, as in Hei 1-198717, raises the problem of increasing the amount of light for illuminating the area outside the visual field because the cylindrical or ball lenses have too strong a refracting power. Moreover, there are problems that the holding of the cylindrical or ball lenses is difficult and that liquids or dust particles are liable to adhere to the cylindrical or ball lenses and the illumination efficiency and light distribution characteristics are degraded because a surface on the exit side of the cylindrical or ball lenses (namely, the entire lens surface from which illumination light emerges) has an irregular shape.